Let’s say you want to cross a river.

There’s no bridge, because it’s 3000 BC and you are a nomadic goat herder. You’ve never seen a bridge that wasn’t just a log over a creek. Crossing a big river is something you haven’t figured out. Maybe nobody has.

You could maybe make a raft, but it’s hard to find suitable wood for that, and you don’t know if it would be safe. Are there man-eating water-lizards lurking out there? Will the current dash you on some rocks before you get across?

You never cross the river.  

Say you want to learn accounting. You’d love to be able to track every penny that comes in and out of your life in a great big ledger. It would satisfy your desire for order and efficiency, and probably save you a lot of money. You could even provide accounting services for businesses in your village.

The problem is it’s 1590, and you are an illiterate tavern owner. There’s maybe one guy in a nearby town who might know about accounting, and the town is eight miles away and you don’t have a horse. Also, that guy is a monk and he has no reason to devote any time to teaching you accounting.